Infectious Disease Testing

The American Red Cross performs laboratory tests for multiple infectious disease markers on every unit of donated blood. Tests are upgraded or replaced with more sensitive technologies as these become available. These tests include:

Trypanosoma cruzi (anti-T. cruzi) (2007)

Chagas disease is a serious, often fatal disease caused by the parasite Trypanosoma cruzi. The agent is endemic in the Americas but most commonly occurs in Latin America. The test used for blood donor screening is a chemluminescent immunoassay (ChLIA) performed on an automated platform (PRISM) for the qualitative detection of antibodies to T. cruzi in human serum or plasma samples. An FDA licensed enzyme strip immunoassay (ESA) is used for confirmatory testing. No approved donor reentry algorithm is yet approved from the Federal Drug Administration. Although T. cruzi may be transmitted to transfusion recipients, to date the Red Cross has not identified any recipients infected by blood components from screened-negative donors, and has not identified any screened-negative donors who subsequently were confirmed positive. All reports of transfusion transmission have been from platelets, or previously (prior to separation of componenets from whole blood) by whole blood from unscreened donors; in total approximately 20 such transmissions are in the scientific literature worldwide. Because donors who test positive have acquired their infection long ago, the Red Cross (and all US blood centers) only test those donors who have not been screened previously.

HBV DNA and HBsAg are the first viral markers to circulate in an individual infected with HBV. Anti-HBc appears in the blood of individuals infected with HBV one to four weeks after the appearance of HBsAg, and at the onset of symptoms for the minority of individuals (5% or less) who develop symptoms. A chemiluminescent immunoassay (ChLIA) is used for the qualitative detection of HBsAg and anti-HBc in human serum or plasma samples. A triplex nucleic acid test (NAT) was introduced by the Red Cross in 2009; it includes the detection of HBV DNA along with the detection of HIV-1 RNA and HCV RNA. Transcription-mediated amplification is the type of NAT used at the Red Cross to detect nucleic acids (RNA and DNA). Testing is performed in mini-pools of 16 individual donation samples. The individual donation samples are tested if contained in a reactive mini-pool followed by virus-specific testing of the reactive individual donation to determine the virus responsible for the sample's reactivity. Specific antigen neutralization is used for HBsAg reactive confirmation; HBsAg-reactive samples that are HBV DNA reactive do not require further testing by neutralization. Anti-HBc reactive samples that are HBsAg and HBV DNA nonreactive by routine testing are further tested individually for HBV DNA using a specialized NAT assay. Donors testing falsely positive for any HBV marker may be reentered. The risk of HBV infection through blood transfusion is between 1 in 800,000 and 1 in 1,000,000 per transfused unit. NAT has reduced the window-period from HBV infection to detection by about 8 to 10 days. This leaves an approximate period of 3 to 4 weeks when an infected donor may not be detected by blood donation screening.

HCV is the causative agent for most, if not all, blood-borne non-A, non-B hepatitis (NANBH). A chemiluminescent immunoessay (ChLIA) is used for the qualitative detection of antibodies to HCV in human serum or plasma samples. A duplex nucleic acid test (NAT) was introduced for HIV-1/HCV RNA detection in 1999, and updated to include the detection of HBV DNA in 2009 (see above). Donors who test HCV-antibody reactive, but NAT negative by routine testing are further tested by another licensed HCV-antibody screening test to determine if the antibody reactivity is specific; donors who test anti-HCV and HCV NAT reactive do not require further testing. Donors testing falsely positive by either antibody or HCV NAT may be reentered. The risk of HCV infection through blood transfusion is about 1 in 1,000,000 per transfused unit. NAT closes the window period between infection and the detection of antibody for those infected with HCV by about 50 to 60 days. This leaves an approximate period of 1 week when an infected donor may not be detected by blood donation screening.

Blood donation screening for HIV-1, the causative agent of AIDS began with antibody testing in 1985. Many improvements in testing have occurred including adding the detection of a second HIV agent (HIV-2 in 1992). A chemiluminescent immunoassay (ChLIA) is used for the qualitative detection of antibodies to HIV-1 and HIV-2. A duplex nucleic acid test (NAT) was introduced for HIV-1/HCV RNA detection in 1999, and updated to include the detection of HBV DNA in 2009 (see above). Donors who test antibody reactive are further evaluated by additional tests to confirm the presence of HIV antibody and to differentiate HIV-1 from HIV-2 antibodies. Tests for this purpose include an HIV-1 indirect immunoflourescence assay, an HIV-2 enzyme-linked immunoassay and an HIV-1 and HIV-2 rapid test. Donors testing falsely positive by either antibody or HIV NAT may be reentered. The risk of HIV-1 infection through blood transfusion is about 1 in 1,000,000 to 1,500,000 per transfused unit. NAT closes the window period between infection and the detection of antibody for those infected with HIV by about 2 weeks. This leaves an approximate period of 7 to 10 days when an infected donor may not be detected by blood donation screening.

Human T-Lymphotropic virus (HTLV-I/II) (1998)

HTLV-I is a human retrovirus that has been associated with neoplastic conditions and a variety of demyelinating disorders. HTLV-II is not yet proven unequivocally to be of significant clinical concern. Qualitative antibody detection for both HTLV-I and HTLV-II in a combined test is performed using ChLIA (as is done for HBV, HCV and HIV). There are no NAT assays available for HTLV-I/II, no licensed supplemental tests to confirm antibody and no protocol approved by the FDA for donor reentry. Donors who test reactive for anti-HTLV-I/II are tested by a series of research assays to determine if antibody is present. The current risk of transfusion-transmitted HTLV-I/II is less than 1 in 2,000,000.

Syphilis (Treponema pallidum) (1950's)

The test used for syphilis is a qualitative screening test that detects the presence of antibodies to the spirochete (cork screw-shaped bacterium) Treponema pallidum. The assay is based on the principle of agglutination and pattern recognition. Confirmation is performed using another serologic test for total antibodies, an enzyme-linked immunoassay, as well as a test for reagin (a protein-like substance that is present during acute infection and for several months following resolution of infection). No cases of transfusion-transmitted syphilis have been recorded for more than 50 years.

West Nile virus (WNV) (2003)

WNV is a flavivirus commonly found in many areas of the world including West Africa, Europe and the Middle East. The virus is most commonly transmitted to humans through mosquito bites; it was first introduced in the US in 1999 and reached epidemic proportions in 2002, the same year that WNV was documented to be transmitted by blood and organs. WNV RNA is detected by NAT using the same type of assay as used for HIV-1, HCV and HBV. Following the introduction of blood donor screening there have been nine cases of transfusion transmission from screened blood; all are due to donations having very low levels of virus. Testing by NAT occcurs in mini-pools like that done for HIV-1, HCV and HBV (see above); however, in addition, measures are in place to reduce the risk of transmissions from donors with low concentrations of virus that would occur during WNV epidemics. This is done by converting from testing donations in mini-pools (which is done for all HIV-1, HCV and HBV NAT) to testing donors individually in areas of on-going outbreaks. Since testing began in 2003 through the end of 2012, the Red Cross has detected approximately 1,600 WNV-infected donors.

Other Testing

In addition to these tests, the Red Cross tests every unit to identify the donor’s blood group (O, A, B or AB) and Rh type and screens for atypical or unusual red cell antibodies. Units tested negative for cytomegalovirus (CMV) are also available. Investigational protocals have been implemented to test for infectious disease agents that are present in certain geographic regions of the US and have been demonstrated to be transfusion transmitted, such as dengue viruses in Puerto Rico and the parasite that causes babesiosis in New England and the upper Midwest.